The polishing pads used for the mirror finishing (for example, chemical mechanical polishing or CMP) of the semiconductor wafers used as substrates for the fabrication of integrated circuits frequently take the form of composite materials of a resin and a velour-quality or suede-quality fabric, or of a relatively soft sheet with a large compressive deformability as yielded by impregnating nonwoven fabric with a thermoplastic polyurethane resin and inducing wet coagulation.
Accompanying higher levels of integration and the increasing use of multilayer wirings, there has been increasing demand in recent years with regard to semiconductor wafers for cost reductions in addition to quality improvements such as higher levels of planarization. In association with these trends, the demands on polishing pads have included long-term usability and higher functionality, for example, the ability to achieve better planarization than to date.
The existing polishing pads that are based on a relatively soft nonwoven fabric have good contact properties with the wafer and also exhibit an excellent polishing slurry retention; however, due to their softness they have an inadequate capacity to planarize the surface to be polished. In addition, polishing slurry and polishing debris clog the void regions in the nonwoven fabric, which results in a tendency for damage to the wafer surface to easily occur. Moreover, cleaning is problematic as the polishing slurry and polishing debris infiltrate the void regions in the nonwoven fabric in depth and a short polishing pad life is thus also a problem.
Polishing pads that use a polymer foam are also known; they are often used in applications that require planarization, such as wafer polishing, because they are more rigid than the nonwoven fabric-type polishing pads. In addition, due to their independent cell structure, polishing pads that use a polymer foam are not subject to the in-depth infiltration into void regions by polishing slurry and polishing debris that is seen with nonwoven fabric-type polishing pads, and because of this polishing pads that use a polymer foam are relatively easy to clean and can also tolerate long-term use. Polyurethane foam is frequently used as the polymer foam in particular for its excellent wear resistance.
Polyurethane foam polishing pads are generally produced by appropriately grinding or slicing a polyurethane foam. The polyurethane foam used for polishing pads has heretofore been produced by foaming and curing a two-liquid curable polyurethane by casting (refer, for example, to Japanese Patent Application Laid-open No. 2000-178374, Japanese Patent Application Laid-open No. 2000-248034, Japanese Patent Application Laid-open No. 2001-89548, and Japanese Patent Application Laid-open No. Hei 11-322878). However, it has been difficult with this method to achieve a uniform reaction and foaming, and there have also been limits with this method on the ability to raise the hardness of the obtained polyurethane foam. In addition, the polishing characteristics, such as the planarity of the polished surface and the planarization efficiency, of conventional polyurethane foam polishing pads may fluctuate, and one cause of this is thought to be nonuniformity in the foam structure in the polyurethane foam that forms the base. Moreover, a polishing pad having a higher hardness is desired in order to raise the planarization efficiency (refer to The Science of CMP, Masahiro KASHIWAGI et al., Science Forum Inc., published 20 Aug. 1997, pp. 113-119).
With regard to high-hardness polishing pads, on the other hand, a polishing pad has been proposed that comprises a polyurethane foam that has a density of 0.5 to 1.0 g/cm3, a cell size of 5 to 20 μm, and a hardness (JIS-C hardness) of at least 90 (refer to Japanese Patent Application Laid-open No. 2002-371154). This polyurethane foam comprises a thermoplastic polyurethane that is obtained by reacting a polymer diol, an organic diisocyanate, and a chain extender and that contains at least 6 weight % nitrogen atom derived from the isocyanate group and has a dynamic viscoelastic modulus E′50 at 50° C. of at least 5×109 dyn/cm2. However, when this polishing pad according to the cited invention containing at least 6 weight % isocyanate-derived nitrogen atom is used, its excessively high hardness creates the risk that a large number of scratches will be generated in the surface of the polished wafer.
The present invention was pursued in view of the circumstances described above and has as an object the introduction of a polishing pad that can achieve an improved planarity for the polished surface and an improved planarization efficiency while producing little scratching. Additional objects are the introduction of a polymer material and a foam obtained from this polymer material. They are useful as the said polishing pad. The present invention in particular has as an object the introduction of a polishing pad that exhibits a stable polishing rate even in the presence of water.